The catalytic oligomerization of olefins is a known technique for manufacturing hydrocarbon basestocks useful as lubricants. Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for many years and have led to the relatively recent market introduction of a number of superior polyalpha-olefin (PAO) synthetic lubricants, primarily based on the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index (VI), while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These new synthetic lubricants lower friction and hence increase mechanical efficiency across the full spectrum of mechanical loads from worm gears to traction drives and do so over a wider range of operating conditions than mineral oil lubricants.
The chemical focus of the research effort in synthetic lubricants has been on the polymerization of 1-alkenes. Well known structure/property relationships for high polymers as contained in the various disciplines of polymer chemistry have pointed the way to 1-alkenes as a fruitful field of investigation for the synthesis of oligomers with the structure thought to be needed to confer improved lubricant properties on them. Due largely to studies on the polymerization of propene and vinyl monomers, the mechanism of the polymerization of 1-alkene and the effect of that mechanism on polymer structure is reasonably well understood, providing a strong resource for targeting on potentially useful oligomerization methods and oligomer structures. Building on that resource, in the prior art oligomers of 1-alkenes from C.sub.6 to C.sub.20 have been prepared with commercially useful synthetic lubricants from 1-decene oligomerization yielding a distinctly superior lubricant product via either cationic or Ziegler catalyzed polymerization.
One characteristic of the molecular structure of 1-alkene oligomers that has been found to correlate well with improved lubricant properties in commercial synthetic lubricants is the ratio of methyl to methylene groups in the oligomer. The ratio is called the branch ratio and is calculated from infra-red spectral data as discussed in "Standard Hydrocarbons of High Molecular Weight", Analytical Chemistry, Vol.25, no.10, p.1466 (1953). Viscosity index has been found to increase with lower branch ratio.
Oligomeric liquid lubricants exhibiting very low branch ratios have not previously been synthesized from 1-alkenes. Oligomers prepared from 1-decene, for example, by either cationic polymerization or Ziegler catalyst polymerization have branch ratios of greater than 0.20. Shubkin. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 15-19, provides an explanation for the apparently limiting value for branch ratio based on a cationic polymerization reaction mechanism involving rearrangement to produce branching. Other explanations suggest isomerization of the olefinic group in the one position to produce an internal olefin as the cause for branching. Whether by rearrangement, isomerization or a mechanism which is yet to be elucidated, it is clear that the production of synthetic lubricants by the oligomerization of alkenes using conventional oligomerization catalysts results in excessive branching with the result that the limits of achievable lubricant properties, particularly the combination of high viscosity index and low pour point are severely constrained. Obviously, increased branching increases the number of isomers in the oligomer mixture, orienting the composition away from the structure which would be preferred for high viscosity although, as acountervailing factor, increased branching leads also to low product pour point, at least from a consideration of the theoretical concepts discussed above.
Recently, novel lubricant compositions comprising polyalpha-olefins and methods for their preparation have been disclosed in U.S. Pat. Nos. 4,827,064 and 4,827,073. The lubricant compositions, referred to in this specification as HVI-PAO, are made by the oligomerization of olefins such as 1-decene over a reduced Group VIB (IUPAC Table) metal catalyst, preferably chromium, and are characterized by a remarkable combination of properties: they possess very high viscosity index (VI) and low pour point. The VI may be as high as 150 or more and is typically 200 or even higher. The pour point (ASTM D-97 or equivalent) remains, however, low being typically below -15.degree. C. in the absence of any pour point improver. Reference is made to U.S. Pat. Nos. 4,827,064 and 4,827,073 for a description for a detailed disclosure of these HVI-PAO materials and their preparation.
The process for preparing the HVI-PAO lubricants comprises, as noted above, contacting a C.sub.6 -C.sub.20 1-alkene feedstock with reduced valence state chromium oxide catalyst on porous silica support under oligomerizing conditions in an oligomerization zone to produce the high viscosity, high VI liquid hydrocarbon lubricant with branch ratios less than 0.19 and pour points below -15.degree. C. The oligomerization temperature is typically maintained at a value between 90.degree. and 250.degree. C. to produce the lubricant viscosity product oligomers. These HVI-PAO lubricants cover a wide range of viscosities and may be used as high viscosity index lubricant basestocks after hydrogenation to remove residual unsaturation. The lubricant viscosity range materials exhibit a remarkably high VI and low pour point even at high viscosity.
By operating the oligomerization process at lower temperatures, higher viscosity materials may be produced and these materials may be used as viscosity index (VI) improvers for lubricants, both of mineral oil and synthetic origin, as described in U.S. patent application Serial No. 07/345,606, filed 1 May 1989 (Mobil Case 5362), to which reference is made for a description of these oligomers and of the way in which they are produced. The higher viscosity HVI-PAO products described in Serial No. 07/345,606 typically have viscosities between 725 and 15,000 cS at 100 C., corresponding to weight molecular weights from about 15,000 to 200,000 and number molecular weights from about 5,000 to about 50,000; carbon numbers for these molecular weights are from about C.sub.30 to about C.sub.10,000, with a preferred range from about C.sub.30 to about C.sub.5,000. These hydrocarbon oligomers are characterized by properties similar to those described in U.S. Pat. Nos. 4,827,064 and 4,827,073, namely, high VI coupled with excellent low temperature fluidity properties including pour point for the liquid products.
In the production of the HVI-PAO oligomers, oligomerization in a fixed bed reactor provides certain economic and process control advantages not readily achievable through other reactor and process configurations. However, the feasibility of fixed bed catalytic processing of 1-alkenes to prepare HVI-PAO lubricants depends upon an effective process for catalyst regeneration that will permit multiple regeneration cycles without significant loss in product yield or diminution of the unusual properties of HVI-PAO lubricants. Previous processes for the polymerization of 1-alkenes using catalysts similar to, although not identical to, the catalyst used in the HVI-PAO process were directed to high polymer preparation using very low concentration of catalyst. Hence, catalyst regeneration was not a requirement for process feasibility.